The invention concerns a method for tunnel microscopy, especially a tunnel microscopy method for recording the magnetic, spin or susceptibility structure of a sample, and a tunnel micro scope (a so-called xe2x80x9cspin-polarized tunnel microscopexe2x80x9d) for implementation of the method as well as methods for use of such a tunnel microscope.
A traditional tunnel microscope, for instance as described by G. Binnig et al. in xe2x80x9cPhys. Rev. Lett.xe2x80x9d, Vol. 49, 1992, p. 57 ff., is shown in the diagram in FIG. 6. The tunnel microscope 10xe2x80x2 is designed for non-contact scanning of a sample 50xe2x80x2 on the basis of the tunnel effect. The tunnel microscope 10xe2x80x2 specifically includes a tunnel tip 20xe2x80x2 with a piezo-electrical drive 21xe2x80x2, a control circuit 30xe2x80x2 and a display and evaluation system 40xe2x80x2. The tunnel tip 20xe2x80x2 typically consisting of tungsten is moved across the surface of the sample 50xe2x80x2 using the piezo-electrical drive 21xe2x80x2 for scanning in the x and y directions. During scanning, the tunnel current between the tunnel tip 20xe2x80x2 and the surface is continuously measured and using the control loop 31xe2x80x2 of the control circuit 30xe2x80x2, the z coordinate of the tunnel tip is set so that the tunnel current remains constant during scanning. The two-dimensional dependency of the z coordinate from the x and y coordinates represents the topography of the sample surface, which may be displayed using the display and evaluation system 40xe2x80x2 and be subject to further image processing. Using a tunnel tip 20xe2x80x2 sharpened in atomic dimensions, for this mapping of the topography, local resolutions below the nm range may be achieved.
Further investigation methods are known which use scanning microscopes, for which for local resolution not the tunnel current, but for instance optical characteristics or electrical field effects are measured at the sample surface. There is specifically also interest in recording the spin structure of a sample, i.e. the magnetic sample characteristics using a local resolution typical to tunnel microscope examination methods. In this respect, in particular, the following three magnetic scanning microscopy techniques are known.
The tunnel microscope known from U.S. Pat. No. 5,266,897 is based on magnetic force microscopy (MFM). The tunnel microscope is operated using a vibrating cantilever tip, which is subject to effects of magnetic forces with reference to the sample surface. The magnetic force between the sample and the cantilever tip is effected depending on the magnetic sample characteristics, so that the distance of the tip to the sample and, therefore, the tunnel current through the cantilever tip changes. This technology has the following disadvantages.
For magnetic force microscopy, the magnetic recording of the structure is based on the local resolution of measurement of force exerted on a magnetic tip by the magnetic stray field of a sample. But the stray field of the sample is not a local surface characteristic. To the contrary, it is created in the sample volume. The local resolution is limited by the so-called xe2x80x9cmagnetic volumexe2x80x9d of the tip. The best local resolution for the MFM method is in the range of 20 to 40 nm. The MFM method is furthermore disadvantageous because contrast formation is effected by magnetization of a layer reaching into the sample volume, and therefore possibly the image of the sample surface is adulterated. Also, MFM is usually operated as non-contacting. Due to the long-range magnetic interaction, the local resolution even for a working distance between tip and sample of some nanometers is limited. Only in case the of provision of specific precautions, for instance as described by P. Grutter et al. in xe2x80x9cJ. Appl. Phys.xe2x80x9d, Vol. 67, 1990, p. 1437 ff., the may local resolution be improved to up to 10 nm.
In the case of magnetooptic near field microscopy (SNOM), the sample surface is also scanned using a detector tip. The detector tip essentially includes a sharp glass fiber which locally resolved measures the magnetooptic Kerr effect at a resolution of below the wavelength of the light used. The local resolution of the magnetooptic SNOM method is defined laterally by focussing of the light field and vertically by the penetration depth into the sample. Up to now, only a local resolution of up to 60 nm has been achieved (refer to C. Durkan et al. in xe2x80x9cAppl. Phys. Lett.xe2x80x9d, Vol. 70, 1997, p. 1323 ff.).
Scanning electronic microscopy with polarization analysis (SEM-PA) is based on recording of the spin polarization of secondary electrons shot out of the top atomic layers of a sample using a scanning electronic microscope. The disadvantage of this method is again the limited local resolution, which is laterally limited by the focus of the primary electron beam and achieves about up to 20 nm (refer to H. Matsuyama et al. in xe2x80x9cJ. of Electron Microscopyxe2x80x9d, Vol. 43, 1994, p. 157 ff.).
From U.S. Pat. No. 4,939,363 (corresponding to EP 0 348 239 A1), a tunnel microscope for investigation of the magnetic characteristics of a sample surface is known, which is partially shown in schematic FIG. 7. The tunnel microscope 10xe2x80x3 specifically comprises the tunnel tip 20xe2x80x3 with piezo-electrical drive 21xe2x80x3 as well as (not shown) the control circuit and the display and evaluation system. Below the sample 50xe2x80x3, a permanent magnet 60xe2x80x3 is arranged, which cooperates with magnetic coils 61xe2x80x3 with reversable polarity as follows. Using the permanent magnet 60xe2x80x3 and the magnetic coils 61xe2x80x3, the tunnel tip 20xe2x80x3 is magnetized in such a manner that a magnetic field forms in axial the direction from the tunnel tip to the sample. On the basis of the so-called xe2x80x9cmagnetic tunnel effectxe2x80x9d (refer to M. Julliere in xe2x80x9cPhys. Lett. Axe2x80x9d, Vol. 54, 1975, p. 225 ff.), the tunnel current depends on the orientation of the spins of the electrons in the sample relative to the spins of the tip. The magneto tunnel effect is based on the dependency of tunnel probability from the energetically split state densities of the electrons in the sample. Locally resolved spin measurement is achieved by performing two tunnel current measurements. Within each case different tip magnetization is performed at each measuring point of the tunnel tip 20xe2x80x3. The spin state at the sample location may be derived from the tunnel current difference for the two orientations of the magnetic field. Even though, using this technology, in comparison with the above stated technologies, theoretically a substantially better local resolution of up to a few xc3x85ngstrxc3x6m may be achieved, the spin-polarized tunnel microscope according to U.S. 4,939,363 is disadvantageous for the following reasons.
For a conventional tunnel microscope, in contrary to the spin-independent microscopy (according to FIG. 6, see above), the z position of the tunnel tip is not controlled so that the tunnel current is constant (xe2x80x9cconstant current modexe2x80x9d). Instead, the tunnel current is in each case measured for the different magnetizations for constant z coordinate. Measurement with variable tunnel current is disadvantageous due to the deviation of the measuring principle from the traditional spin-independent microscopes. Also, signal evaluation with great effort must be performed to separately derive the topographical properties and the magnetic properties from the measured current values. This evaluation is performed subsequently using numerical means so that additional time is required for recording the surface image. An important disadvantage furthermore consists in the fact that for the magnetic tunnel effect the dependency of the tunnel current on the magnetization direction is substantially lower than the dependency of the tunnel current from the distance tip-sample. Because the dependency of magnetization only amounts to about 20% of the dependency on topography, when recording a tunnel image of the surface of a ferromagnet using a magnetic tip, the magnetic domains indeed become visible in the tunnel image. But it is almost completely blanketed by the topographic contrasts. Finally, the function of the conventional spin-polarized tunnel microscope is limited by the fact that the tunnel tip 20xe2x80x3 (see FIG. 7) consists of EuS-coated tungsten or nickel, which due to its relatively high coercive force requires strong fields also having an effect on the sample. In addition, due to change in magnetization due to the large magnetostriction, geometrical changes of the tip occur, which have an interfering effect on the tunnel current. When using a EuS coated tip, the tip must be cooled down to cryogenous temperatures, which forms a substantial further disadvantage.
The object of the invention is to provide a new and improved scanning microscope method by which magnetic properties, especially the spin structure, of a sample surface may be recorded, with high local resolution in the range of atomic dimensions, with improved precision and increased speed of image recording. The object of the invention also consists in providing a scanning microscope by which such a method may be implemented.
According to a first aspect of the invention, a method for tunnel microscopy is provided for which the surface of a sample is scanned for each individual position using a tunnel tip and locally resolved tunnel current measurement is performed, whereby, during scanning periodic remagnetization of the tunnel tip is performed using a predetermined remagnetization frequency (f) and locally resolved signal components are derived from the tunnel current (It) or a z coordinate of the distance between the tunnel tip and the sample or a value derived from it, which occur at the remagnetization frequency (f) and are characteristic for magnetic sample properties, and imaging of the magnetic structure of the sample surface is performed on the basis of the derived signal components, whereby the tunnel tip is not subject to geometrical change during scanning and the signal components measured with local resolution are characteristic for the magneto-tunnel resistance between the sample and tunnel tip and the imaged magnetic structure of the sample surface includes the local spin polarization properties of the sample surface.
According to a further aspect of the invention, a scanning microscope is provided, which comprises a tunnel tip, which may be moved across the surface of a sample in an evacuated sample chamber using a piezo-electric drive, a control circuit for control of the tunnel tip, a display and evaluation system for display and processing of surface properties of the probe, a magnetization device for remagnetization of the tunnel tip at a predetermined remagnetization frequency (f) and a phase-sensitive amplifier, designed for phase-sensitive amplification of the tunnel current (It) or the z coordinate of the tunnel tip or of values derived from them at the remagnetization frequency (f) or a multiple of it, whereby the magnetization device at least includes a coil and an oscillator, which is adapted for applying electrical currents to the coil, which change direction at the magnetization frequency (f), and the tunnel tip at least partially consists of a material having such a low saturation magnetostriction that for remagnetization essentially no geometrical changes of the tunnel tip occur.
The invention is specifically based on the idea to provide a scanning microscope with a magnetizable tunnel tip and to record the magnetotunnel resistance between the tunnel tip and the sample surface by means of a differential detection method, by means of periodic remagnetization of the tunnel tip during scanning of the sample surface using a specific remagnetization frequency (f) and deriving signal components from the tunnel current, a tip-sample distance coordinate (z coordinate) or a value derived from this, which occur at the remagnetization frequency (f) and which are characteristic for the sample properties. Preferably, phase-sensitive analysis is performed to record tunnel current changes at the frequency of periodic remagnetization (magnetic reversal). This change in tunnel current corresponds to the magnetic contrast of the sample surface which is independent of the topographical contrast and which is recorded simultaneously with it. Using the method according to the invention, in deviation from traditional MFM methods, not the magnetic stray field of a sample, but the local spin polarization on the basis of the magnetotunnel effect is measured.
The method according to the invention is preferably implemented in xe2x80x9cconstant current modexe2x80x9d. This means that as for the traditional tunnel microscope according to FIG. 6, provision is made during scanning of the sample surface in the x and y directions that the z coordinate of the tunnel tip is set so that the tunnel current is constant in time average. The frequency of remagnetization according to a preferred embodiment is greater than the cut-off frequency of the control circuit for setting the z coordinate. According to a modified embodiment, a remagnetization frequency is also possible which is smaller than the cut-off frequency of z tracking. In the first case, remagnetization is performed so fast that the drive system is not able to react to remagnetization and, therefore, the tunnel current is modulated with the desired variation according to the magnetic structure of the sample at the remagnetization frequency. In the second case (low remagnetization frequency), the tunnel tip could be moved in case of changes of the tunnel current dependent on the spin polarization. In the case of phase-sensitive analysis of the z coordinate of the tunnel tip, a signal characteristic for the spin structure of the sample surface is in turn detectable.
A scanning microscope according to the invention is specifically characterized by a tunnel tip being able to be remagnetized and a magnetizing device for the tunnel tip. The tunnel tip consists of a material which depend in g on the application is more soft magnetic than the material of the sample. The material of the tunnel tip is selected in such a manner that the tunnel tip does not show a hysteresis effect when remagnetized and remains geometrically-mechanically unchanged. For this purpose, the material of the tunnel tip preferrably has a low coercive force (for instance below 1 oerstedt respectively 3 mA/cm) and a low saturation magnetostriction (for instance below 2*10xe2x88x927).
An important characteristic of the invention concerns the properties of the tunnel tip. The tunnel tip is a stiff component which is operated free of vibrations. It has the shape of a stiff tip or a stiff pin with a sharp end. In deviation from the cantilever tip of traditional tunnel microscopes, the tunnel tip of the tunnel microscope according to the invention is not deformed during operation, specifically during remagnetization. The stiff mount of the tunnel tip in the tunnel microscope enables the highly sensitive tunnel current measurement for recording of spin polarization. The tunnel tip is not changed by the stray field of the sample.
The invention has th e following advantages. Using the combination of spin-dependent detection of electrons on the basis of differential magnetotunnel resistance using scanning microscopy, atomic local resolutions below 10 nm up to a few Angstrxc3x6m are achieved. The high local resolution results from the fact that an intrinsic value of the surface (spin polarization) is recorded. The great local resolutions are better by magnitudes than the corresponding parameters of the MFM, SNOM or SEMPA methods. Therefore, for the first time, examinations of the spin structure of condensed matter with atomic resolution (for instance anti-ferromagnets, ferromagnets and/or magnetic composite materials) or of micro-magnetic structures (domains and domain walls) are possible with practically acceptable measuring periods (1/ms per point) and precision. For contrast formation, only the top atomic layers of the sample surface are important. This is advantageous in basic research, for instance for examination of surface processes, and also for applications in data storage technology.